1. Field of the Invention
The present invention relates to a drive system for a vibration motor which utilizes a mechanical vibration wave by electromechanical energy conversion means such as an electrostrictive element or a magnetostrictive element.
2. Description of the Prior Art
As taught in U.S. Pat. No. 4,019,073, a vibration motor converts a vibration motion generated when a periodic voltage is applied to an electrostrictive element to a rotational motion or a one-dimensional motion. Since it needs no wiring unlike a conventional motor, it is simple in construction and compact, provides a high torque even at a low rotating speed and has a low inertia rotation.
However, the known vibration wave motor transduces the standing wave vibration motion generated in the vibration member to a unidirectional motion of the movable member by friction-driving the movable member such as a rotor which contacts to the vibration member
In order to reverse the direction of the motion, a mechanical structure for changing a contact position or a contact angle between the vibration member and the movable member is necessary. Accordingly, in order to forwardly and reversely rotate the vibration wave motor, a large scale device is needed and a characteristic feature of the vibration wave motor, that is, the simple construction and the compactness is lost.
In order to resolve the above problem, a vibration wave motor driven by a travelling vibration wave has recently been proposed. A structure of such a vibration wave motor is shown in FIG. 1. A vibration absorber 4, a metal ring vibration member 2 having electrostrictive elements 3a and 3b bonded on a surface facing the absorber 4 and a movable member 1 are inserted in this order in a central cylinder 5a of a stator 5 which serves as a base, and the stator 5, the absorber 4, the electrostrictive elements 3a and 3b and the vibration member 2 are mounted not to rotate with respect to each other. The movable member 1 is press-contacted to the vibration 2 by its gravity or urging means, not shown, to maintain the integrity of the motor.
A plurality of electrostrictive elements 3a are arranged at a pitch equal to one half of a wavelength .lambda. of the vibration wave and a plurality of electrostrictive elements 3b are also arranged at a pitch of .lambda./2. The electrostrictive elements 3a (or 3b) may be a single element polarized at the pitch of .lambda./2. The electrostrictive elements 3a and 3b are phase-differentially arranged at a mutual pitch of (n.sub.0 +1/4).lambda. where n.sub.0 =1, 2, 3, . . . Lead wires 7 are connected to the electrostrictive elements 3a and lead wires 9 are connected to the electrostrictive elements 3b, and they are connected to an AC power supply 6 and a 90.degree. phase shifter 8, respectively (see FIG. 2). A lead wire 10 is connected to the metal vibration member 2 and it is connected to the AC power supply 6.
A friction area 1a of the vibration member 1 is formed by a hard rubber to enhance a friction force and reduce abrasion, and it is press-contacted to the vibration member 2.
FIG. 2 shows a generation of the vibration wave of the motor. The electrostrictive elements 3a and 3b bonded to the metal vibration member 2 are shown adjacently for the sake of convenience but they meet the requirement of .lambda./4 phase shift and are essentially equivalent to the arrangement of the electrostrictive elements 3a and 3b of the motor shown in FIG. 1. Symbols .sym. in the electrostrictive elements 3a and 3b indicate that they expand in a positive cycle of the AC voltage and symbols .sym. indicate that they shrink in the positive cycle.
The metal vibration element 2 is used as one electrode to the electrostrictive elements 3a and 3b, and an AC voltage V=V.sub.0 sin .omega.t is applied to the electrostrictive elements 3a from the AC power supply 6 while an AC voltage V=V.sub.0 (.omega.t.+-..pi./2) which is shifted by .lambda./4 is applied to the electrostrictive elements 3b from the AC power supply 6 through the 90.degree. phase shifter 8. Signs + and - in the equation are switched by the phase shifter 6 depending on the direction of motion of the movable member 1 (not shown in FIG. 2). When the sign + is selected, the phase is shifted by +90.degree. and the movable member 1 is moved forwardly, and when the sign - is selected the phase is shifted by -90.degree. and the movable member is moved reversely. Let us assume that the sign - is selected and the voltage V=V.sub.0 sin (.omega.t-.pi./2) is applied to the electrostrictive elements 3b. When only the electrostrictive elements 3a are vibrated by the voltage V=V.sub.0 sin .omega.t, a standing wave vibration is generated as shown in FIG. 2(a), and when only the electrostrictive elements 3b are vibrated by the voltage V=V.sub.0 sin (.omega.t-.pi./2), a standing wave vibration is generated as shown in FIG. 2(b).
When the two AC voltages having the phase shift are simultaneously applied to the electrostrictive elements 3a and 3b, the vibration wave travels. FIG. 2(a) shows a waveform at time t=2n.pi./.omega., FIG. 2(b) shows a waveform at time t=.pi./2.omega.+2n.pi./.omega., FIG. 2(c) shows a waveform at time t=.pi./.omega.+2n.pi./.omega. and FIG. 2(d) shows a waveform at t=2.pi./2.omega.+2n.pi./.omega.. A wavefront of the vibration wave travels in the x-direction.
Such a travelling vibration wave includes a longitudinal wave and a lateral wave. Looking at a mass point A of the vibration member 2 as shown in FIG. 3, a longitudinal amplitude u and a lateral amplitude w create a counterclockwise rotating elliptic motion. The movable member 1 is press-contacted to the surface of the vibration member 2 (arrow P) and contacts only an apex of the vibration plane. Thus, it is driven by the longitudinal amplitude components of the elliptic motion of the mass points A, A', . . . at the apex and is moved in a direction of an arrow N.
A velocity of the mass point A at the apex is V=2.pi.fu (when f is a vibration frequency) and a velocity of the movable member 1 depends on it and also depends on the lateral amplitude w because of the friction drive by the press contact. Thus, the velocity of the movable member 1 is proportional to the magnitude of the elliptic motion of the mass point A and the magnitude of the elliptic motion is proportional to the voltage applied to the electrostrictive elements.
In starting such a vibration wave motor, a long time is required to start up the motor. When the travelling vibration wave is generated by a drive start signal to change status from a stop status to an operation status, the elliptic motion gradually grows by the vibration energy of the electrostrictive elements in a transient period before a steady state, and after a certain time period, it becomes steady. That is, the certain time period is required before the vibration energy of the electrostrictive elements which is initially zero is converted to the longitudinal vibration energy and the lateral vibration energy of the stable travelling vibration wave.
Further, since the movable member 1 is in press-contact to the vibration member 2 even when the electrostrictive elements do not vibrate, a large force is needed to manually move the movable member 1.
It is an object of the present invention to provide a drive system for a vibration wave motor which can improve a start-up characteristic.
It is another object of the present invention to provide a drive system for a vibration wave motor which generates a standing vibration wave in a manual operation mode to improve an operability.
Other objects of the present invention will be apparent from the description of the preferred embodiments.